One of the most challenging targets is to deliver genetic materials to cells safely. The identification of defective genes responsible for disease states, either through defective control of gene expression, which leads to overproduction or underproduction of key proteins, or the production of defective proteins, offers new possibilities for the treatment of disease. By controlling the defect at the genetic level, a range of diseases could potentially be treated effectively rather than by merely treating the symptoms of these diseases.
The use of genetic material to deliver genes for therapeutic purposes has been practiced for many years.
Generation of therapeutic gene products (such as polypeptides, proteins, mRNA and RNAi) by expression of therapeutic gene product-encoding DNA in transformed cells has attracted wide attention as a method to treat various mammalian diseases and enhance production of specific proteins or other cellular products. This promising technology, often referred to as gene therapy (Crystal et al., Science 1995, 270, 404), is generally accomplished by introducing exogenous genetic material into a mammalian patient's cells. Transformed cells can be accomplished by either direct transformation of target cells within the mammalian subject (in vivo gene therapy) or transformation of cells in vitro and subsequent implantation of the transformed cells into the mammalian subject (ex vivo gene therapy) (for reviews, see Chang et al. 1994 Gastroenterol. 106:1076-84;). The introduced genetic material can be designed to replace an abnormal (defective) gene of the mammalian patient (“gene replacement therapy”), or can be designed for expression of the encoded protein or other therapeutic product without replacement of any defective gene (“gene augmentation”). Because many congenital and acquired medical disorders result from inadequate production of various gene products, gene therapy provides means to treat these diseases through either transient or stable expression of exogenous nucleic acid encoding the therapeutic product. Although the initial motivation for gene therapy was the treatment of genetic disorders, it is becoming increasingly apparent that gene therapy will be useful for the treatment of a broader range of acquired diseases such as cancer, infectious disorders (such as AIDS), heart disease, arthritis, and neurodegenerative disorders such as Parkinson's and Alzheimer's diseases.
DNA is inherently an unstable material in an active biological environment where many specific enzymes capable of degrading and metabolizing DNA are found (Ledoux et al., Prog. Nucl. Acid Res., 1965, 4, 231). In addition, natural protection against alien DNA exists in the body. Thus, the gene therapy, antisense oligonucleotide therapy and gene vaccination approaches described above require that the DNA and DNA analogues would survive in such a hostile biological environment and in addition, that the DNA and DNA analogs would penetrate biological barriers, be taken up into cells and be delivered to the correct subcellular compartment to exert their therapeutic effects. While some DNA is taken up naturally into cells, the amount taken up is typically small and inconsistent, and expression of added DNA is therefore poor and unpredictable.
A number of strategies have been proposed to achieve delivery of DNA into living cells. These include the use of liposomes (Fraley et al., Proc. Natl. Acad. Sci. USA, 1979, 76, 3348), cationic lipids (Felgner et al., Proc. Natl. Acad. Sci USA, 1987, 84, 7413), and the use of cationic polymers, or polycations, such as polylysine and polyornithine as DNA delivery agents (Farber et al., Biochim. Biophys. Aca, 1975, 390, 298 and Wagner et al., Proc. Natl. Acad. Sci. USA, 1990, 87, 3410).
Therapeutic approaches that involve intervention at the gene level are widely recognized as promising technologies, these methods are limited by the absence of an efficient and reliable method of delivering DNA and RNA.
There is thus a widely recognized need for, and it would be highly advantageous to have, a novel delivery system for delivering therapeutic agents such as DNA and RNA molecules into living cells, which would overcome the present limitations associated with gene therapy.